Plastic film wrap is commonly used in a number of commercial applications, particularly the packaging of goods for shipment. For example, plastic film wrap can be employed to bind together single units, or to secure a number of units to pallets or the like. Typically, goods are stacked on a pallet and secured thereto with straps; a continuous winding of a sheet of plastic film; or a combination of straps and plastic film.
It is known that the utility of plastic film for the purpose of securing goods can be enhanced by stretching the film to near its yield point and then permitting the film to relax slightly. Stretching reduces the thickness of the film, and provides a greater length of film for wrapping. The resulting stretched film exhibits an increased tensile strength and a "memory", or tendency to contract toward its unstretched length when stretching tension is removed. For example, a stretched film with a 10% memory will shrink 10% of its stretched length when it relaxes as it is wound about a pallet load. This shrinkage assists in securely holding the palletized goods onto the pallet under compression thereby decreasing the shifting of goods on the pallet during transport.
Stretched film can be wrapped around a pallet load either manually or automatically. Manual devices for stretching plastic film as it is wound about a pallet load, such as described in U.S. Pat. No. 4,166,589, include hand actuated braking mechanisms to stretch the film as it is wound about the pallet load from a roll. Such devices have several disadvantages, including a need for sufficient physical strength on the part of the person wrapping, to stretch the film, and the likelihood of uneven stretching tension being applied to the film as it is wrapped. It is also known to provide a plastic film stretching device as a component of an automated wrapping machine, as is described in U.S. Pat. No. 5,040,356. However, such an arrangement increases the cost, intricacy and size of the automated wrapping device.
Providing rolls of pre-stretched film overcomes many of the disadvantages of the above-described devices. Typically, pre-stretching involves passing a sheet of plastic film through a series of staggered rollers having different diameters and/or different rotational velocities such that the sheet is tensioned lengthwise by a predetermined amount and, consequently, is stretched. Conventionally, after passing through the rollers, the tension on the film is lessened to permit the film to relax slightly, and the stretched film is wound onto a core for later use. Film can be pre-stretched after its initial manufacture, as described in U.S. Pat. No. 5,531,393, or as part of the film manufacturing process wherein molten plastic is formed into a film, cooled, stretched as above, and wound onto a core for later dispensing.
Generally, the core is a hollow plastic or fiberboard tube which helps to maintain the shape of the roll. A hollow core permits the roll to be mounted on a spindle to unwind the film from the roll. There are a number of disadvantages associated with the use of cores in rolls of film wrap. Fiberboard cores, especially those intended for large commercial rolls of plastic film, have to withstand handling and the crushing forces of the film which is rolled upon them under tension. As a result, the cores are generally of heavy, thick fiberboard, with a high glue content for added rigidity. Depending on the size of the roll, the cores can have a weight ranging from approximately 0.5 to 2 kg (1 to 3 lbs.), can have an outer diameter of up to 10 cm (4 in), and can have a thickness of up to 2.5 cm (1 in.). Such cores form a significant portion of the total radial dimension and weight of each roll of film. Thus, the cores increase the cost of shipping rolls of film by decreasing the number of rolls which can be packed and shipped in a container, and increasing the weight of the shipment. Further, the cores used in conventional plastic film rolls are relatively expensive and can account for up to a fifth of the final price of a roll of film. Also, the high glue content in the fiberboard cores makes them unsuitable for recycling, with the consequence that they must be discarded in a landfill site, or in other non-environmentally friendly manners.
Additionally, due to the memory of the film, pre-stretched film has a tendency to shrink and compress around a rigid core by the same percentage it would compress about a pallet load. Over time the multiple layers of film forming the roll tend to fuse together making it difficult to unroll the film for later use. One method of overcoming the tendency of the pre-stretched film to fuse together is described in the above-mentioned U.S. Pat. No. 5,531,393, where a method of producing a roll of pre-stretched plastic film includes a step of embossing the film with a textured roller as it is being stretched. As the embossed film is wound on a core, the embosses trap air between the layers of film, separating the layers and preventing their fusing. However, this solution still requires that the film be wound onto a core.